"In a notice sent to the company, U.S. Department of Transportation regulators alleged a chain of bad decisions by Exxon leading up to the spill and in its immediate aftermath. That included Exxon employees' failure to close an upstream safety valve, which could have significantly reduced the size of the spill after it was first detected.

As a result, the agency said, oil continued gushing into the flooding river for almost an hour after the break was noticed by pipeline controllers in Houston."